This invention relates to square root circuits such as those used with differential pressure transducers in the measurement of flow. The purpose of such a circuit is to provide a reading on a measuring instrument which is directly related to flow when the measurement is derived from a differential pressure transducer.
In the prior art analog circuits for this purpose have been constructed using high gain amplifiers with a squaring circuit inserted in the negative feedback path so that the output produced is proportional to the square root of the amplifier input. With such a circuit the adjustment of the zero has been accomplished by disconnecting the squaring circuit and then varying a signal added to the input so that with zero flow the reading obtained is zero.
The disconnection of the squaring circuit in the feedback has been necessary because of a problem which arises when the amplifier output is small. With such an output, the negative feedback signal developed by the squaring circuit has a very flat characteristic. Thus, with changes in input in the zero output region, there is very little change in output of the squaring circuit and a great change in output from the amplifier. Zeroing is therefore virtually impossible with the squaring circuit connected and in control of the feedback. Therefore manual switching to remove the squaring circuit is resorted to with the inevitable problems of manual operation, such as the operator forgetting to reconnect the squaring circuit.
In addition to the zeroing problem, it is difficult at low flows for the operator to determine if the circuit is operable, for he might not observe changes in the readings of the magnitude he would normally expect. Although accurate readings in the low flow range may not be important, such unexpected changes lead to a lack of confidence in the measuring system.
It is an object of this invention to provide a square root circuit which will make it possible to accurately zero the measurement being made by the circuit and provide a smooth change in output as input approaches zero.